Positive displacement vacuum pumps are used in various applications. For example, a double vacuum pump apparatus composed of two positive displacement vacuum pumps connected in series may be used to carry out pressure swing adsorption (PSA) as a gas purification method.
In the PSA, for example, an adsorption tower filled with an adsorbent for adsorbing impurities is used. In the gas purification by the PSA using such an adsorption tower, for example, the following cycle including an adsorption step and a depressurization regeneration step is repeated in the adsorption tower. In the adsorption step, a source gas, which is a mixed gas, is introduced into the adsorption tower whose interior is in the state of a relatively high pressure, and impurities contained in the source gas are adsorbed by the adsorbent while the non-adsorbed gas is emitted from the adsorption tower. The non-adsorbed gas is a gas enriched with a target gas, and is obtained as a purified gas. In the depressurization regeneration step, while the interior of the tower is depressurized to a relatively low pressure, impurities are desorbed from the adsorbent, and the desorbed gas containing the impurities is emitted to the outside of the tower. Positive displacement vacuum pumps may be used in order to depressurize the interior of the adsorption tower in this depressurization regeneration step.
Such positive displacement vacuum pumps are described, for example, in Patent Documents 1 and 2 below.    Patent Document 1: JP H10-296034A    Patent Document 2: JP 2006-272325A
According to the disclosures in these publications, two positive displacement vacuum pumps (blowers) are connected either parallel to or in series with the adsorption tower according to variations of the load (the pressure in the adsorption tower) during the depressurization of the adsorption tower. For this reason, it is necessary to perform control to switch between the parallel connection and the serial connection, and the setting of the switching timing is not easy. Also, in these publications, no consideration is made as to what type of control needs to be performed to minimize the total power consumption of the two vacuum pumps in operating the two vacuum pumps. Furthermore, although flow vibration caused by the pulsation of exhaust gas of positive displacement vacuum pumps is involved, no consideration is also made as to how to avoid the adverse effect of the vibration on the on-off valve disposed downstream of the vacuum pumps in these publications.